Arm movements when rotating a spherical object were experimentally investigated. Twelve volunteers participated in the experiment and were asked to rotate a sphere for a large range of amplitude. Results showed that subjects anticipated their posture at the beginning of object manipulation even for low rotation amplitudes. The way of anticipation strongly depended on rotation direction. The end-state comfort hypothesis, effects of joint limits and principle of minimum work were examined for explaining motion control. The anticipation would ensure a better end-state comfort while avoiding joint limits in case of higher amplitude of object rotation. Meanwhile, it should not deteriorate the comfort at the beginning of manipulation too much. High postural variability for low rotation amplitude tasks suggested that there might exist a range of postures of similar level of comfort. These findings will be useful in developing human behaviour-based motion simulations for digital human.
Practitioner summary: Arm movement was investigated when rotating a spherical object with a large range of amplitude. The end-state comfort hypothesis, effects of joint limits and principle of minimum work were examined for explaining motion control. Results will be helpful for a better design of rotary controls and for developing motion simulation algorithms.